The present invention relates to a process for preparing a superconducting thin oxide film. More particularly, the present invention relates to a process for preparing a high-quality thin film of an oxide superconductor by a laser ablation method.
Among the various methods that can be used to prepare thin films, a laser ablation method which requires no electromagnetic fields of any kind, is considered to be suitable for preparing thin films of high quality.
While various types of lasers can be used as light sources, an excimer laser ablation method that uses an excimer laser as a light source is capable of preparing thin films at high rate and yet at low substrate temperature since the laser beam used has high energy in a short wavelength range from 150 nm to 400 nm. This method has the following additional advantages and has atracted the attention as a technique adapted for preparing thin films of oxide superconductors which are multi-component compounds:
(1) a thin film having no compositional mismatch from the target can be obtained; PA1 (2) film formation can be effected over a broad range of pressures up to about 1 Torr; PA1 (3) the growth rate can be increased to an extremely high level; and PA1 (4) the reaction involved is a non-thermal equilibrium process under illumination with high-energy beam.
However, the excimer laser involves pulsed oscillation and the duration of its light emission is usually very short on the order of 10-odd nanoseconds. Therefore, in the laser ablation method which uses such an excimer laser as a light source, the target is irradiated with intermittent pulsed laser beam.
In general, in the laser ablation method, particles or vapor are emitted from the target only for the period when the target is irradiated with laser beam. The emitted particles or vapor undergo reaction at the substrate surface to be deposited as a thin film on the surface. Having a high energy with an extremely short pulse width, the excimer laser can generate a large amount of particles or vapor per pulse of laser beam. Hence, a very rapid growth rate can be attained by increasing the number of pulses but, at the same time, insufficient crystallization may occur to deteriorate the quality of deposited film.
Another factor to be considered in forming superconducting thin oxide films is the film forming temperature (usually measured as the substrate temperature) which is typically as high as about 700.degree. C. and above. To prevent the thermal deterioration or reaction of the substrate or substrate material used in thin film formation, it is desired to lower the film forming temperature. Particularly in the case where superconducting thin oxide films are applied to electronics engineering, lowering the film forming temperature is an essential technique. However, at lower film forming temperatures, the performance of superconducting thin oxide films will usually deteriorate to such a level that they have no commercial value.
As mentioned above, film formation occurs in the excimer laser ablation only for the period while pulsed laser beam is applied to the target and the particles or vapor being emitted from the target surface are kept supplied onto the substrate. Thus, in this method of film formation, a thin film is formed by successively depositing extremely thin layers (molecular layers) made of the particles or vapor sputtered from the target by respective pulses of laser beam.
The atoms and molecules of the particles or vapor that have reached the growing surface of the substrate will not be immediately stabilized and are subject to reaction or continued movement or diffusion at the atomic and molecular levels. The magnitude of such reaction, molecular movements and diffusion will eventually decrease to lie within a certain range to form a stable thin film but the time to this stabilization will vary with the energy supplied such as the substrate temperature and the energy of laser beam.
Therefore, a high-quality thin film of oxide superconductor can be prepared by properly selecting these conditions, as well as the pulse rate of pulsed laser beam and the average rate of film formation.